Acetate and Triacetate

The cellulose esters triacetate and acetate are formed through acetylation of cotton linters or wood pulp using acetic anhydride and an acid catalyst in acetic acid. At this point the cellulose is fully acetylated. 

Although limited hydrogen bonding occurs between molecules within acetates, triacetate is incapable of forming hydrogen bonds. Van der Waals forces caused by the interaction of adjacent acetate and triacetate chains are the major associative forces between the acetylated cellulose molecular chains. The average number of acetylated anhydroglucose units in chains of acetates and triacetates usually varies between 250 and 300 units. 

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Fig. 5. The effect of dry heat exposure on acetate and triacetate fibers. Tested at 65% rh, 21°C after exposure. A, acetate, 100°C B, triacetate, 130°C and C,...

The ratio of stress to strain in the initial linear portion of the stress—strain curve indicates the abiUty of a material to resist deformation and return to its original form. This modulus of elasticity, or Young s modulus, is related to many of the mechanical performance characteristics of textile products. The modulus of elasticity can be affected by drawing, ie, elongating the fiber environment, ie, wet or dry, temperature or other procedures. Values for commercial acetate and triacetate fibers are generally in the 2.2—4.0 N/tex (25—45 gf/den) range. 

The wet modulus of fibers at various temperatures influences the creasing and mussiness caused by laundering. Figure 6 shows the change with temperature of the wet modulus of acetate and triacetate, and compares them with a number of other fibers (2). Acetate, triacetate, and rayon behave quite similarly, with a lower sensitivity than acryUc. 

The abihty of a fiber to absorb energy during straining is measured by the area under the stress—strain curve. Within the proportional limit, ie, the linear region, this property is defined as toughness or work of mpture. For acetate and triacetate the work of mpture is essentially the same at 0.022 N/tex (0.25 gf/den). This is higher than for cotton (0.010 N/tex = 0.113 gf/den), similar to rayon and wool, but less than for nylon (0.076 N/tex = 0.86 gf/den) and silk (0.072 N/tex = 0.81 gf/den) (3). 

Table 2. Tenacity and Elongation of Commercial Acetate and Triacetate Fibers...

Acetate and triacetate exhibit moderate changes in mechanical properties as a function of temperature. As the temperature is raised, the tensile modulus of acetate and triacetate fibers is reduced, and the fibers extend more readily under stress (see Fig. 4). Acetate and triacetate are weakened by prolonged exposure to elevated temperatures in ah (see Fig. 5). 

Electrical Behavior. The resistivity of acetate varies significantly with humidity with typical values ranging from 10 ohm-cm at 45% rh to 10 ohm-cm at 95% rh (16). Because of the high resistivity both acetate and triacetate yams readily develop static charges and an antistatic finish is usually apphed to aid in fiber processing. Both yams have also been used for electrical insulation after lubricants and other finishing agents are removed. 

Chemical Properties. Under slightly acidic or basic conditions at room temperature, acetate and triacetate fibers are resistant to chlorine bleach at the concentrations normally used in laundering. 

Acetate and triacetate are essentially unaffected by dilute solutions of weak acids, but strong mineral acids cause serious degradation. The results of exposure of heat-treated and untreated triacetate taffeta fabrics to various chemical reagents have been reported (9). Acetate and triacetate fibers are not affected by the perchloroethylene dry-cleaning solutions normally used in the United States and Canada. Trichloroethylene, employed to a limited extent in the UK and Europe, softens triacetate. 

Acetate and triacetate polymers are white amorphous soHds produced in granular, flake, powder, or fibrous form. They are used as raw materials in the preparation of fibers, films, and plastics. Polymer density varies and ranges from 100 kg/m for the fibrous form to 500 kg/m for granules. Acetate polymer is shipped by trailer tmck, railroad freight car, or multiwaH bags. 

Extrusion Processes. Polymer solutions are converted into fibers by extmsion. The dry-extmsion process, also called dry spinning, is primarily used for acetate and triacetate. In this operation, a solution of polymer in a volatile solvent is forced through a number of parallel orifices (spinneret) into a cabinet of warm air the fibers are formed by evaporation of the solvent. In wet extmsion, a polymer solution is forced through a spinneret into a Hquid that coagulates the filaments and removes the solvent. In melt extmsion, molten polymer is forced through a multihole die (pack) into air, which cools the strands into filaments. 

Many ceUulosic derivatives form anisotropic, ie, Hquid crystalline, solutions, and cellulose acetate and triacetate are no exception. Various cellulose acetate anisotropic solutions have been made using a variety of solvents (56,57). The nature of the polymer—solvent interaction determines the concentration at which hquid crystalline behavior is initiated. The better the interaction, the lower the concentration needed to form the anisotropic, birefringent polymer solution. Strong organic acids, eg, trifluoroacetic acid are most effective and can produce an anisotropic phase with concentrations as low as 28% (58). Trifluoroacetic acid has been studied with cellulose triacetate alone or in combination with other solvents (59—64) concentrations of 30—42% (wt vol) triacetate were common. 

Yarns and Fibers. Many different acetate and triacetate continuous filament yams, staples, and tows are manufactured. The variable properties are tex (wt in g of a 1000-m filament) or denier (wt in g of a 9000-m filament), cross-sectional shape, and number of filaments. Individual filament fineness (tex per filament or denier per filament, dpf) is usually in the range of 0.2—0.4 tex per filament (2—4 dpf). Common continuous filament yams have 6.1, 6.7, 8.3, and 16.7 tex (55, 60, 75, and 150 den, respectively). However, different fabric properties can be obtained by varying the filament count (tex per filament or dpf) to reach the total tex (denier). 

Staple is produced by cutting a crimped tow into short lengths (usually 4—5 cm) resembling short, natural fibers. Acetate and triacetate staple are shipped in 180—366 kg bales, but production is quite limited. Conventional staple-processing technology appHed to natural fibers is used to process acetate and triacetate staple into spun yam. 

Fig. 11. World production of cellulose acetate and triacetate fibers I, cigarette tow B, textile (74).

A hst of world acetate and triacetate producers is given ia Reference 74. The combiaed annual world acetate production (filament, staple, and tow) peaked ia 1980 with 672,000 t, dropped to 574,000 t ia 1984, and rose to 731,000 t ia 1991. The United States accounted for ca 45% of the world total. Other principal acetate produciag countries iaclude the UK, Japan, Canada, Italy, and the former USSR. 

Textiles. A unique combination of desirable quaUties and low cost accounts for the demand for acetate ia textiles. In the United States, acetate and triacetate fibers are used ia tricot-knitting and woven constmctions, with each accounting for approximately half the total volume. This distribution changes slightly according to market trends. The main markets are women s apparel, eg, dresses, blouses, lingerie, robes, housecoats, ribbons, and decorative household appHcations, eg, draperies, bedspreads, and ensembles. Acetate has replaced rayon filament ia liner fabrics for men s suits and has been evaluated for nonwoven fabrics (79—81). 

Other Cellulosics. Rayon is bleached similarly to cotton but under milder conditions since the fibers are more easily damaged and since there is less colored material to bleach. Cellulose acetate and triacetate are not usually bleached. They can be bleached like rayon, except a slightly lower pH is used to prevent hydrolysis. The above fibers are most commonly bleached with hydrogen peroxide. Linen, dax, and jute requite more bleaching and mil der conditions than cotton, so multiple steps are usually used. Commonly an acidic or neutral hypochlorite solution is followed by alkaline hypochlorite, peroxide, chlorite, or permanganate, or a chlorite step is done between two peroxide steps. A one-step process with sodium chlorite and hydrogen peroxide is also used. 

Cellulose acetate and triacetate fibres are brightened with disperse-type FBAs, including derivatives of 1,3-diphenylpyrazoline (11.19). These form a commercially important group of FBAs. If suitably substituted they can be applied to substrates other than acetate and triacetate. The commercially more important products of this type are used to brighten nylon and acrylic fibres. Their preparation and other aspects of pyrazoline chemistry are discussed in section 11.8. Examples of pyrazolines used to brighten acetate and triacetate... 

Reaction of 1-nitropropane with glutaraldehyde in aqueous ethanol in the presence of sodium hydroxide yields a mixture of two products, the major component of which, lr-ethyl-l-nitrocyclohexane-2c,6f-diol (98), can be isolated in 36% yield ). Acid-catalyzed acetylation converts (98) into the di-O-acetate, hydrogenation yields the corresponding amine, which has been characterized as the hydroacetate, N-acetate and triacetate. Configurational assignments followed from NMR data, which clearly showed the steric non-equivalence of the two hydroxyl groups vicinal to the tertiary center. 

Cellulose acetate and triacetate may be used as plastics or spun into fibers for textiles. They are made by the reaction of cellulose with acetic anhydride. 

Acetate fiber is the generic name of a fiber that is partially acetylated cellulose. They are also known as cellulose acetate and triacetate fibers. They are nontoxic and generally non-allergic, and so are ideal from this aspect as clothing material. 

The manufacture of acetate staple and tow follow the same scheme as employed for continuous filament yams, except that the yarns are combined as they leave the spinning tube to form a tow which is mechanically crimped and cut into staple lengths. Acetate and triacetate arc packaged in hales of about 4(X) pounds (181 kiltigrains). 

Mechanical Properries. Acetate and triacetate have a tenacity in the range of (1.10-0.12 N/tex tl.l-1.4 gl/den 1 with a breaking elongation of about 25-30%. Compared to other common textile libers, acetate and triacetate are relatively weak. e.g.. 20-25 3 the tenacity of polyester. This is not necessarily a disadvantage, because fabric construction can bo used to obtain the desired fabric performance targets. Pilling, the accumulation of fuzz balls on the fabric with wear, is not a problem as It is with the higher tenacity fibers. 

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